This invention relates generally to electromechanical structures and in particular to electromechanical structures having a deformable element, the structure being integratable onto an integrated circuit.
Many types of electromechanical structures have been incorporated into integrated circuits. For example, the concept of a structure comprising a sealed cavity formed using semiconductor processing techniques with a suspended membrane whose deformation or deflection allows its use as a sensor or actuator is known in the literature. U.S. Pat. Nos. 4,744,863 and 4,853,669 granted to Guckel et al., U.S. Pat. Nos. 5,321,989 and 5,431,057 granted to Zimmer et al., and U.S. Pat. No. 5,316,619 granted to C. H. Mastrangelo discloses some form of this approach to realise pressure sensors. In all known cases, including the above, the sensor is a planar structure in parallel to the silicon substrate which typically forms the bottom plate of the sensor.
In the patents listed above, a pressure sensing diaphragm is formed using a deposited layer onto the chip surface. The use of silicon over a sacrificial buried oxide layer to create micromechanical elements is also known in the literature, such as that described in U.S. Pat. No. 5,677,560, granted to Zimmer et al. which etches out a buried oxide to create micromechanical structures. This structure uses the active silicon layer above the buried oxide to create the micromechanical diaphragm.
In all of the prior art described above, the plane of the deformeable element (the diaphragm) is parallel to the surface of the integrated circuit.
Known implementations of micromechanical structures that are fabricated in the same plane as the surface of the integrated circuit (e.g. diaphragm-type pressure sensor) consume considerable surface area on the integrated circuit. In addition, known methods of implementing sensors on silicon are highly process dependent. Accordingly, a need arises for an electromechanical structure that occupies minimal surface area, and can be easily scaled into different process generations.
These needs and others are addressed by the electromechanical structure of the present invention. The electromechanical element of the present invention includes a deformeable element. In one embodiment, the invention provides an electromechanical structure being a transducer that realises a mechanical diaphragm in silicon. The structure can be incorporated onto an integrated circuit manufactured through a standard semiconductor manufacturing process. The diaphragm can be used as a sensor or actuator.
The deformeable element is formed in a plane perpendicular to the surface of the integrated circuit by etching one or more trenches into the silicon. In the case where two parallel trenches are etched the silicon xe2x80x9cwallxe2x80x9d between the two trenches forms a diaphragm. The top of one of the trenches is sealed under vacuum conditions to form an evacuated cavity while the other trench is left open. The silicon xe2x80x9cwallxe2x80x9d now has a vacuum cavity on one side and the other side is open to ambient pressure.
This wall forms a xe2x80x9cdeflectablexe2x80x9d or xe2x80x9cdeformeablexe2x80x9d diaphragm of a sensor. Ambient pressure changes deflect the diaphragm, and the deflection is proportional to the pressure difference between the ambient pressure and the vacuum cavity. The deformation of the diaphragm can be measured directly as a variation in the electrical characteristics of the structure, thereby serving as a sensor. Alternatively, the diaphragm can be excited electrically to serve as an actuator. In the case where both trenches are sealed under vacuum conditions the silicon wall between the two can be excited electrically to serve as a resonator or mechanical filter.
In the case where the etch forms a single column of silicon surrounded by a trench the trench can be sealed under vacuum and the column can be contacted electrically. This sealed column can be used as a resonator or mechanical filter.
A main advantage of the structure arises from the fact that the deformeable element is substantially perpendicular to the surface of the integrated circuit. This allows large deformeable elements to be manufactured while consuming very little surface area of the integrated circuit. This approach is very cost effective and will scale with all process technologies.
Accordingly the invention provides an integrated circuit comprising:
a substrate having an upper surface defining a first plane;
an electromechanical element including a deformeable element defining a second plane;
such that the first and second planes intersect.
The first and second planes are preferably substantially perpendicular with respect to one another.
The deformeable element is typically insulated from the upper surface of the substrate.
In one embodiment the deformeable element is preferably responsive to applied pressure, said applied pressure effecting a change in the electrical characteristics of said electromechanical element.
In another embodiment the deformeable element can be electrostatically actuated to create a resonator or mechanical filter.
In one embodiment, the deformable element is a transducer diaphragm forming a first wall of an evacuated cavity, an outer portion of the first wall being exposed to ambient pressure conditions and an inner portion of the first wall being exposed to an evacuated cavity, such that changes in ambient pressure with respect to the evacuated cavity effects a deflection of the diaphragm in the vertical plane, such deflection being electrically measurable.
The transducer diaphragm preferably forms a first wall of an evacuated cavity, the cavity having a second wall, the second wall of the cavity being electrically insulated from the first wall, such that on application of a signal between the first wall and the second wall the diaphragm is actuated, the actuation of the diaphragm resulting in the diaphragm vibrating mechanically.
The frequency of the diaphragm vibration typically is modulated by the frequency of the applied signal.
The invention also provides a method of forming an electromechanical structure onto an integrated circuit comprising the steps of:
forming an evacuated cavity in an active device layer of the integrated circuit, and wherein a wall of the cavity or a column within the cavity is substantially perpendicular to the substrate of the integrated circuit and is deformable.
The deformation of the wall of the cavity or the column within the cavity is desirably actuated by
application of a signal to electrically excite the side wall or column, or
having one side of the wall being exposed to a sealed cavity and the other side responsive to ambient pressure such that any changes in ambient pressure effect a deformation of the side wall.
The method preferably further comprising the steps of electrically isolating the electrochemical structure from the remaining integrated circuit.
By sensing any deformation of the wall resultant from fluctuations in pressure between the ambient pressure and the evacuated cavity, the method of the invention may also be used to form a pressure sensor on an integrated circuit.
The application of a signal between the deformable side wall and a second wall of the evacuated cavity, or between a column within the cavity and the wall of the cavity may be used to actuate the deformable side wall or column. This actuation results in the vibration of a wall or column, the characteristics of the vibration being defined by the applied signal and the mechanical dimensions of the vibrating wall or column, and may be used to form an actuator or resonator on an integrated circuit.
In accordance with one embodiment of the present invention, a method of forming an evacuated cavity in the active device layer of the integrated circuit comprises the steps of:
i) etching a trench in the device layer, the trench being etched in a plane substantially perpendicular to the substrate,
ii) filling the trench with a sacrificial material,
iii) forming a cover over the filled trench, and
iv) etching the sacrificial material previously deposited from the trench, and sealing the trench.
The column resonator of the present invention is formed in a plane perpendicular to the surface of the integrated circuit by etching in a pattern that creates a standing silicon column surrounded on all sides by a trench. The trench surrounding the column can be sealed in a similar manner to the previous embodiments to form an isolated column standing in a vacuum cavity supported at the top and bottom only.
In this case, a filled isolation trench is formed in the silicon outside the etched trench, and encloses it completely. This allows outside walls of the trench to be electrically biased with respect to the silicon column, allowing the column to be actuated and resulting in the column vibrating mechanically.
These and other features and advantages of the present invention will be better understood with relation to the accompanying drawings.